Medical image recording methods require a multiplicity of processing steps for the images of an object to be examined that are generated mostly by a detector/source system. In this case, the detector/source system records projections of the radiation attenuated by the object with reference to different angles. These projections can be used to derive in the overlap region of the projections a so-called reconstruction volume that is a precondition for the backprojection into a three-dimensional volume image data record of the object.
The basis of the medical image recording methods is provided here by image projection and image transformation methods that combine the recorded projections on the basis of the orientation of the detector/source system during a projection recording with the orientation-dependent attenuation of the respective radiation by the object to be examined to form a reconstruction volume, and subsequently backproject it as images. The spatially dependent attenuation values thus determined then serve as basis for the backprojection into the volume image data record. In order to record the projections of the object to be examined, such as, for example, the body of a patient, radiation sources and correspondingly arranged radiation detectors are frequently arranged for transirradiating the object. The object to be transirradiated, or subregions of the object, characteristically attenuate the radiation emitted by the radiation source, characteristic projections of the attenuation profile respectively being determined in the radiation detector as a function of the relative position and distance of the radiation source and the radiation detector in relation to the object.
A line integral can respectively be formed relative to the object for each beam profile on the basis of the known angular dependence of the detected projections by means of the so-called Radon transformation. In the overlap region of the projections, the line integrals form the reconstruction volume that serves as database for a subsequent backprojection into the three-dimensional volume image data record.
The backprojection of the reconstruction volume in relation to a volume image data record is frequently performed by way of a so-called inverse Radon transformation. The Fourier slice theorem or a filtered backprojection is frequently used for this purpose. The filtered backprojection is mostly used in medical image recording systems because of the high numerical stability. The medical images can subsequently be extracted from this volume image data record thus generated, doing so with reference to freely selectable image planes.
For the purpose of completely covering a three-dimensional image data record, it is necessary, moreover, to determine inside the overlap region the data points that are not covered by the overlapping of at least two projections in an overlap region.
Currently, the missing data points inside the overlap region are interpolated by way of additional projection data from additional detectors on the basis of the projection data of the largest detector. Only in the case of complete coverage of the overlap region of all the beam profiles is a filtered backprojection of the projections of the attenuation profiles possible. Moreover, it is necessary to take account in the case of the backprojection of the type of projection, for example a parallel projection or a fan projection.
In order to generate an interpolated reconstruction volume, recording the projections therefore frequently requires a particularly high rotational speed for the detector/source system rotating by 360° or 180° on a circular path, in order to obtain a complete image of the object to be examined within a short time period. In particular, pictures of moving organs of a patient such as, for example, pictures of the lung or of the heart, must be recorded within a very short time interval, since otherwise the movement of the organ would distort the medical images. The conventional approach is to use a number of detector/source systems fitted in an offset fashion in the direction of rotation in order to generate the projections during a revolution, and subsequently to combine the projections by way of image projection and image transformation methods, and to backproject them into a three-dimensional volume image data record.
During the generation of the projections, in commonly used image recording systems two detector/source systems move about the object to be examined, the relative position of the two systems in relation to one another remaining the same during the recordings. For reasons of cost and space, it is often customary to give the second and the further detectors smaller design dimensions than the main detector of the first detector/source system such that the overlap region of all the beam profiles of the two detector/source system does not detect the entire object to be examined, or the overlap region thus produced is smaller than in the case of detectors of a multidetector/source system that have the same dimensions.
An additional complicated factor here is that a mathematical stipulation of the filtered backprojection of the reconstruction volume enables a complete overlapping of all the beam profiles only when all the data points of the reconstruction volume are determined. In particular, in the case of multidetector/source systems with different detector sizes, the problem arises of an overlap region that is reduced and covered by projections only incompletely in part by comparison with multidetector/source systems of the same detector size.
The problem is currently solved by determining the projections with the aid of a two-fold detector/source system, the first detector/source system having a large detector, and the second detector/source system having a smaller detector. While taking account of the projections of the first detector/source system, the data points inside the reconstruction volume of the additional detector/source systems are interpolated by considering adjacent line integrals inside the overlap regions covered only partially by projections. The reconstruction volume thus mathematically completed is subsequently completely backprojected into a volume image data record by way of a backprojection. No comparison currently exists with measured data for the inadequately covered data points of the only partially covered overlap regions.
Subsequently, it is optionally possibly after completion of the abovedescribed image processing method to carry out filtering and image reprocessing of the image points in the reconstruction volume and/or of the medical images thus generated.
Thus, DE 198 42 944 A1 describes a method for the reconstruction of a three-dimensional image of an object scanned in the course of a tomosynthesis, as well as an apparatus for the tomosynthesis. By selecting two- and three-dimensional filter functions for the reconstruction volume by taking account of suitable weighting elements and optimized filters, the invention mentioned there provides the possibility of improving the image quality and—for the slicewise display of the volume image data record—of reducing high frequency image components.
In the case of all the reconstruction methods known in the prior art, it is disadvantageous for projections that, in order to completely cover the reconstruction volume, mathematical interpolations of data points are undertaken on the basis of adjacent line integrals in the reconstruction volume that can lead during the backprojection to an impairment of the image quality of the volume image data record, as in computed tomography (CT), for example.